Since the early 1980s, companies with large data processing requirements have been using interfaces such as RS-232, V.35, RS-530, and X.21 to move large volumes of data between devices. As the companies expanded, it became necessary to move the data between facilities using modems or expensive leased lines (T1/DS3). In order to do this, the DTE-generated data was sent between sites using DCE devices such as modems or CSU/DSU units. These modems or CSU/DSUs generated clocks internally that then were sent to the DTE devices for data clocking in both directions (counter-directional timing).
As data speeds increased, clock/data skew became an issue using counter-directional clocking (DCE generated TC/RC). Co-directional timing was introduced in which both the DTE and DCE devices generated their own clocks in phase with the associated data.
As data infrastructure grew, it became necessary for 24/7 availability of data paths, so crosspoint matrix switches were developed to allow paring of DTE and DCE devices, as well as monitoring capabilities to diagnose issues. These matrix switches passed the data and clock signals without affecting the integrity or synchronicity of the signals.
Now that IP networks are widespread throughout large companies and the military, an edict has been given to migrate legacy interfaces to IP. In order to reduce cost by forklift upgrading, the CESoP technology (Circuit Emulation Services over Packet) was introduced that allowed users to place these devices between their equipment in lieu of the existing leased lines. Since the matrix connectivity is still required, the issue becomes synchronizing the clocks between the network endpoints. Traditionally, the CESoP device would derive its clocking from a common clock or generate a master internal clock on one side of the network and use adaptive clock recovery on the other. This works fine as long as the legacy devices are DTEs operating in counter-directional timing mode where all subrate clocks are derived from the master CESoP device clock.